In an effort to increase the productivity of plants and food crops, plant breeders generally develop cultivars that contain certain desirable characteristics such as increased height, growth rate, higher yields, etc. One of the ways in which this may be accomplished is by infusing desirable characteristics into a plant to form a superior plant line. Superior lines are then combined to form an F.sub.1 hybrid that contains the desirable characteristics. Such superior hybrids can be developed in numerous ways.
One popular way of producing superior hybrids is by using male sterility in one of the plants for which hybridization is desired. Male sterile lines allow the breeder to produce hybrid seed more economically by controlling cross-fertilization in the flower of a plant. Cross-fertilization can be controlled by preventing the female parent from self fertilizing. Self-fertilization is eliminated by making the plant male sterile. If the plant is male sterile, then no pollen can be produced for fertilization. Once rendered male sterile, the plant may then be hybridized with a gene donor plant possessing the desired characteristics.
One way to effectuate male sterility is through the use of cytoplasmic male sterility. Present belief is that genetic factors controlling cytoplasmic male sterility (CMS) are found in the cytoplasm, particularly in the genes of the mitochondrial DNA.
Three of the most common cytoplasmic male sterilities in the Brassica species are:
1) Ogura male sterile cytoplasm of Raphanus sativus; PA1 2) Polima male sterile cytoplasm of Brassica napus; and PA1 3) Nap male sterile cytoplasm of Brassica napus.
In Brassica, cytoplasmic male sterility can be transmitted by crossing. The female (egg) parent contributes the cytoplasm, therefore, crossing to CMS females produces CMS progeny. The nuclear genes however are heterozygous. Therefore, six to eight generations of "backcrossing" are necessary to produce a CMS line breeding homozygous for nuclear characters. As an alternative, cytoplasmic male sterile lines can also be produced by protoplast fusion. In protoplast fusion, a protoplast from a plant having commercially desirable traits is combined with a protoplast of a CMS line. The nuclear material of the CMS line is either removed or inactivated prior to fusion so it donates only the cytoplasm. The resulting cytoplasmic hybrid (or cybrid) possesses the CMS cytoplasm and is male sterile. For example, U.S. Pat. No. 5,254,802 discloses B. oleracea plants that contain the Ogura CMS cytoplasm. These plants were obtained by protoplast fusion.
Polima CMS cytoplasm has been used to produce CMS in varieties such as winter-type oil seed rape (Brassica napus) (See Barsby et al., Plant Science, 53: 243-248 (1987)). However, one significant problem with the expression of cytoplasmic male sterility by the polima CMS cytoplasm is that the polima cytoplasm is influenced by environmental conditions. Fan, Z et al. Can. J. Plant Sci. 66:221-227 (1985). More specifically, male sterile plants containing polima CMS cytoplasm are known to become fertile under high temperatures in the field. Id. See also Fu, T. D., Encarpia Cruciferea Newsletter 6: 6-7 (1981).